This invention relates to detection, characterization and study of flaws, and has particular relationship to flaw detection by echo ranging of acoustic energy and by holographic processing. Acoustic flaw detection is uniquely useful for the detection of flaws in metal. It has found extensive applicability in the detection of flaws in pressure vessels of nuclear reactors. For this purpose a high-resolution ultrasonic imaging system has been developed for defining the dimensions and orientation of the flaws. In flaw detection acoustic energy scans the work or the specimen subject to flaw inspection.
In one mode of sonic or acoustic imaging, acoustic energy is focused at points of the scanning pattern on or near the surface. With the acoustic energy so focused the area under observation is all, of a part, of the surface of the work on which this acoustic energy impinges, or any plane parallel to this surface within the work. The acoustic energy impinging on the work at each point produces a broad beam pattern diverging from the point. A C-scan image is derived from this mode of scanning. A C-scan image is an image of the surface, or of any plane within the work perpendicular to the direction of propagation of the scanning acousting energy. A flaw constitutes a discontinuity in the work and a wave of the expanding beam which encounters such a discontinuity is reflected as an echo. A display of the echo pattern is produced from the response of a sensor to the echos. Typically, the acoustical echo pattern may be mixed with an electrical analog of an acoustical reference wave to produce an acoustical interference pattern, i.e., a hologram. The hologram may be reconstructed by a laser beam into a recognizable optical image.
In another mode of acoustic imaging the acoustic energy is focused along a line which scans the work and at each point is propagated into the work. The imaging produced with such scanning is referred to as line-focused imaging. The acoustic energy impinging on a flaw at any point along the line is reflected as an echo. Such a scan may produce a series of what is referred to as B-scan acoustic images. A B-scan is a scan in a plane parallel to the direction of propagation of the acoustic energy. The definition of line-focused images is comparable to that produced holographically as described above. The images derived from the line-focused energy may be displayed on a cathode ray oscilloscope or may be stored in a memory for later display. The line-focused acoustical energy may also be used to produce a C-scan image by selecting echo intelligence received only from a predetermined depth in the work.